This invention relates generally to fiber optic gyroscopes (gyros) and particularly to a closed loop fiber optic gyro having a signal processing arrangement which features reduced sensitivity to gyroscopic drift.
It is well established that closed loop fiber optic gyro signal processing can be achieved through ramp phase modulation. Arrangements for accomplishing this are described in the following references: Progress In Optical Fiber Gyroscopes Using Integrated Optics, Lefevre, et al, 37th Meeting of the Electromagnetic Wave Propagation Panel of the Advisory Group for Aerospace Research and Development (NATO), Istanbul, Turkey, Sep. 23-27, 1985; Double Closed-Loop Hybrid Fiber Gyroscope Using Digital Phase Ramp, Lefevre, et al, Proceedings of the Conference on Optical Fiber Sensors, Optical Society of America, Feb. 11-14, 1985; and Utilization of Fiber Optic Gyros in Inertial Measurement Units, Matthews, The Institute of Navigation Proceedings of the National Technical Meeting, Jan. 23-25, 1990.
The present invention will be described with reference to a "minimum reciprocal configuration" fiber optic gyro which acts as an optical interferometer. The desired optical phase shifts in the interferometer are derived from two sources: (1) the Sagnac phase shift from a rotation rate imposed on a fiber optic coil through which clockwise and counterclockwise light beams travel; and (2) phase shifts imposed by a phase modulator.
The invention uses square wave phase modulation in order to create an output error signal. A square wave is applied to an optical phase modulator. This generates phase signatures for the clockwise and counterclockwise light beams. The difference between the two signatures is applied to a phase difference-intensity transfer function to determine output intensity. If the Sagnac phase shift is constant, the phase difference between the clockwise and counterclockwise beams alternates between two distinct values. These values relate to operating points on a phase difference-intensity transfer function curve. Spikes in the output waveform are a result of tracing out the portion of the transfer function between two operating points. This occurs every (.tau.) seconds, during the fast transitions between operating points. Ignoring the spikes, the peak-peak output square waveform has an amplitude that directly measures the net non-reciprocal phase shift between the clockwise and counterclockwise light beams.